9074 Background: T-VEC is an HSV-1 derived oncolytic immunotherapy designed to selectively replicate in tumors, produce GM-CSF, and enhance systemic antitumor immune responses. We describe factors associated with clinical outcomes in 436 pts with previously treated or untreated, unresected stage IIIB-IV MEL in OPTiM, a randomized (2:1) phase 3 trial of intralesional T-VEC vs subcutaneous GM-CSF. Methods: Multivariate analyses (MVAs) and Cox regression were used to evaluate association of BTB (sum of products of 2 largest perpendicular diameters [per investigator] of measurable lesions at baseline), stage (IIIB/C/IVM1a vs IVM1b/c), treatment (tx) line (TL; 1st vs ≥ 2nd), and ECOG PS (0 vs 1), previously reported to be associated with clinical outcomes, with overall response rate (ORR), durable response rate (DRR; continuous PR or CR ≥ 6 months), and overall survival (OS). Results: 288 T-VEC and 126 GM-CSF pts had available BTB; all received tx as randomized. Median BTB: 14.8 cm2 (interquartile range, 5.8-3...